This application claims the priority of German Patent Application, Serial No. 101 20 832.4, filed Apr. 27, 2001, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.
The present invention relates, in general, to an electromotive servo drive for adjusting a control element.
Servo drives involved here are primarily used in heating, air-conditioning and ventilation systems, for opening and closing control elements such as flap valves, Hereby, the normal basic position may also be an intermediate position. Associated to the electromotive adjustment drive is a control unit. Some applications may require an opening or closing of the control element, such as in case of a danger, e.g. adjustment of the flap valve in the event of fire.
Conventional electromotive servo drives use drive motors having a rotor disposed within a fixed stator. The drive motor has an output journal and a pinion which is wedged on the output journal in fixed rotative engagement. The downstream drive train is hereby so configured that the rotation speed of its output member is significantly smaller than the rotation speed of the output journal of the drive motor. A locking device is provided to maintain the control element in proper end position. A drawback of conventional servo drives is the disposition of the locking device within the drive train and the considerable brake moment as a consequence of the reduced rotation speeds.
Many conventional servo drives are configured as so-called spring-return mechanisms, i.e. a spring assembly, for example a spiral spring, is tensioned when the motor is operated to implement the adjustment, so that the return movement of the control element is effected by the restoring force of the spring assembly. Servo drives of this type are relatively bulky, rendering their installation in some cases prohibitive because the installation space is oftentimes very narrow.
It would therefore be desirable and advantageous to provide an improved electromotive servo drive which obviates prior art shortcomings and is compact in structure, while still being reliable in operation.
According to one aspect of the present invention, an electromotive servo drive includes a drive motor having an external rotor, a drive train in driving relationship with the drive motor and having an output member operatively connected to a control element to be adjusted, and a locking device having a brake element movable between a release position in which the rotor is free to rotate and an operative position in which the rotor is locked in place, a reset spring operatively connected to the brake element for maintaining the rotor at a standstill, when the reset spring is under tension, and a manually-operated actuating shaft for tensioning the reset spring and moving the control element to a base position.
The present invention resolves prior art shortcomings by configuring the locking device to act on the rotor of the drive motor, i.e. at a location in which the respective torque of the entire drive train is the smallest so that the brake moment to be applied is minimized. In addition, the servo drive becomes hereby compact. As the rotor is held by hand when the reset spring is tensioned, the base position of the control element can be easily and precisely established. Adjustment of the base position of the control element can be implemented in a particularly simple way by attaching at least one wheel of the drive train on the actuating shaft and by tensioning the reset spring through turning of the actuating shaft, when the rotor is at a standstill. As the actuating shaft is accessible from outside, the actuating shaft becomes, in effect, a driving element that moves the following part of the drive train in order to move the control element in driving relationship with the output member of the drive train into the intended end position. The drive train is therefore configured to maintain the rotor of the drive motor and possibly the following wheels up to the actuating shaft at a standstill. Suitably, the wheel is supported on the actuating shaft via a freewheel or an overrunning clutch. In this way, the output member of the drive train and the control element can be adjusted, while the rotor stands still. In addition, the freewheel or overrunning clutch allows the rotor during return run, as realized by the reset spring, to coast, when the output member of the drive train strikes against a stop. Therefore, there is no abrupt halting that would require application of great forces.
According to another feature of the present invention, the actuating shaft may be supported through frictional engagement in a bushing which thus conjointly moves, when the actuating shaft is rotated, wherein stops are provided for limiting a rotation angle of the bushing in both rotation directions.
According to another feature of the present invention, the locking device may include a lever, which is supported by the bushing, and a reset spring, which has one end linked to the lever and another end linked to the bushing. Thus, the lever is rotated via the reset spring in a same rotation direction as the actuating shaft. As the lever is part of the locking device, the brake member stays in contact with the rotor of the motor. The rotation angle of the conjointly moved bushing is greater than the rotation angle of the lever, so that the difference between the rotation angles is commensurate with a pressing force by which the brake member is urged against the rotor via the reset spring, whereby the brake member rotates in relation to the lever.
According to another feature of the present invention, the brake member may be configured as a double-armed lever having first and second arms extending at an angle to one another, e.g. an acute angle. As a result, the brake member is elastic to ensure a smooth and soft bearing against the rotor of the drive motor. In addition, when the tension of the reset spring is released during reverse movement, the rotor is ensured to run freely during the start-up phase. The elasticity of the brake member can further be enhanced, when the arms of the brake member are interconnected by a spring element, preferably an annular spring element.
When the rotor is not decelerated, the brake member suitably strikes against a stationary stop. To maintain a sufficient distance of the contact surface of the brake member in relation to the circumferential area of the rotor, the pivoting movement of the brake member in direction toward the stop is assisted by a spring element, e.g. a spring bar. Suitably, the spring element is tensioned when the brake member is moved toward the rotor and applies the thus stored restoring force to move the brake member back to the release position. Instead of a spring bar, it is, of course, also possible to use a tension spring having one end attached to the lever of the locking device or the brake member and another end attached to a fixed component.
According to another feature of the present invention, a coupling element may be mounted in the connection between the actuating shaft and the rotor and so configured to allow a coasting of the rotor when rotated, as the spring element relaxes and the lever mechanism strikes against a stop, and ultimately resulting in a halting of the rotor. This coupling element may be disk-shaped and connected with a drive wheel which is associated to the drive motor.
Release of the locking device by hand is necessary for certain applications. Thus, according to another feature of the present invention, the actuating shaft may be provided with a spring element, e.g. a wrap spring, for urging the actuating shaft to seek a rotation in opposition to a rotation to realize a tensioning of the reset spring so as to release the locking device. Use of a wrap spring is currently preferred because the wrap spring does not lock in one direction, while acting as a catch in the other direction. Suitably, the spring element is disposed in a gap between the actuating shaft and the bushing. When releasing the locking device by suitably turning the actuating shaft, the freewheel in the drive train allows the rotor of the drive motor to freely rotate until it halts and comes to a standstill. Jerky loads are thus prevented.
Of course, tensioning of the reset spring may not only be realized by manually turning the actuating shaft but the use of an electric tool may also be conceivable. To prevent the reset spring from damage as a result of overrotation, the servo drive may be provided with an overload safety mechanism. The overload safety mechanism is so configured as to allow transmission of a certain torque and may include two coupling parts so constructed as to prevent movement of one coupling part, when a predetermined torque is exceeded. As interacting coupling parts, the overload safety mechanism may include a toothed disk and a spring washer having several radial spring legs with angled ends for engagement in peripheral spaces between teeth of the toothed disk. When the angled ends of the spring jump the teeth, an acoustic noise is triggered to indicate to the operator to terminate a rotation of the actuating shaft.